The present invention relates to time division multiplexing of data channels and in particular to time division multiplexing and switching of bit-interleaved digital communication channels by optical techniques. The present invention is particularly applicable to communications switching systems in the core communication network.
There is considerable interest today in ultra-high speed optical transmission. For example, 100 Gigabit per second Ethernet is considered to be the next big leap in transmission and networking from the metro to the core or backbone network. Even with the application today of wavelength division multiplexing, also know as WDM, which increases the data transmission capacity of an optical fiber by one or two orders of magnitude, there is interest in transmitting ultra-high bit-rate channels for economic as well as networking reasons. First, reducing the number of interfaces, or ports, in the core network at the wavelength level may bring economic benefits. Second, the development of new applications and networking practices at the edge of the core network result in the need for containing ultra-high bit-rate data transmission on a single channel. From an historical perspective, the data channel bit-rate adopted for transmission in the core network has always been the highest possible technology can support in an economic way. Today that the rates envisioned are 100 Gbit/s or higher, both electronic and optical techniques have been proposed for multiplexing starting from lower bit-rate tributaries, such as 10 Gbit/s or 40 Gbit/s. This multiplexing in time is called Time Division Multiplexing, also known as TDM. When optical techniques are used to multiplex to these ultra-high bit-rates, the term used is Optical Time Division Multiplexing, also known as OTDM. Specifically, the employment of OTDM techniques allows bit-rates of hundreds of Gigabit per second to be reached on a single wavelength channel.
However, efficient networking in the core network dictates the switching in space—for example between different fibers—of both the wavelengths and the lower bit-rate tributaries, of which these higher bit-rate wavelengths consist of. While several OTDM techniques have been proposed to multiplex lower bit-rate tributaries to ultra-high bit-rates and, conversely, de-multiplex ultra-high bit-rates to lower bit-rate tributaries, the problem of switching tributaries in space using optical techniques has not so far been properly addressed. One reason is because an efficient utilization of transmission bandwidth dictates that space switching of tributaries also involves switching in time, which is to say that tributary channels are individually delayed in the time domain to avoid collision, or in other words to avoid overlap, before they are multiplexed back to ultra-high bit-rate channels. FIG. 1 shows an example prior art arrangement for switching tributaries in space and time. This architecture which involves in sequence Time-Space-Time switching, also known as T-S-T, is in use today and is employed by electronics. With reference to FIG. 1, suppose that the higher bit-rate channel consists of four tributaries. A frame, consisting of four time-slots, or slots, each belonging to one of the four tributaries, of such a higher rate channel is shown 4 for the higher bit-rate channel which is input at 6. The repetition in time of said frame composes the higher data channel. A second channel of the same nominal higher rate is input at 12. A frame of that channel is shown 10, and it includes one empty slot 8, which means that no information is carried in 8. Suppose that slot 2, needs to be switched to the empty slot 8. As shown in this example, slot 2 has to be delayed by one slot. Therefore, space switching alone does not suffice. Space switching alone would create a collision of 2 with slot 14, used by the corresponding tributary. Therefore Time-Slot Interchange, also known as TSI, has to be employed. Depending on the characteristics of the space switch 18, TSI 16 may be required before the space switch in order to avoid collision, also known as blocking, inside the space switch, as well as after the space switch 20, so that the time slots are re-ordered as required by subsequent network elements. The combination of space and time switching, which was illustrated above with the example of the generic T-S-T architecture, is usually referred to as Time Multiplexed Space switching, also known as TMS switching. It is noted that each time-slot may consist of a multiple sequence of bits belonging to the corresponding tributary occupying the said slot. In the special case where each slot consists of a single bit of information, the higher rate channel consisting of a number of such slots is called Bit-Interleaved Time Division Multiplexed channel, and the scheme of multiplexing and switching in space and time a number of such channels is called Bit-Interleaved Time Division Multiplexing. Another definition relates to the so called frame integrity. When the data content of a time-slot within a frame is only permitted to switch to another time-slot of the same frame the condition is called frame integrity. Conversely, when there is no such restriction, meaning that it is acceptable for the data content of one time-slot to be switched to another time-slot of a different frame, the condition is called non-frame integrity. It is also noted that, typically, in TMS switching implementations all input channels have to be in synchronism. With reference to FIG. 1, this means that all inputs such as those numbered 6 and 12, have to be synchronized, or time aligned, with a common clock. In the special case of bit-interleaved time division multiplexing, time alignment more accurate than the bit period, or bit duration, is required.